As an optical medium, glass (optical glass) used to be used. The optical glass is chemically stable and has properties such as superior transparency, formability and hardness. Therefore, it is used to optical waveguide such as an optical fiber, lenses, and electronic parts and so on. However, recently plastic materials become an attractive option to form the optical medium. The optical plastic material utilizes a phenomenon in which light entering into polymer passes through the polymer while repeating reflection at the surface of the polymer. The optical plastic material has high transparency, and superior lightweight and workability than those of the optical glass. The optical plastic material is applied to various techniques. For example, there is a plastic optical fiber (POF) used as the optical fiber.
The POF is formed from polymers each of which has refractive index different to each other. Light entering to one end of the POF passes through the POF while repeating total reflection at the interface between layers having different refractive indices. Recently, a graded-index POF, in which the refractive index varies in radial direction thereof, has gotten a lot of attention. In the graded-index POF, light through the center and light entering the periphery are transmitted in approximately same time by its specific refractive index distribution. Therefore, distortion does not occur in an input signal, which realizes large capacity of transmission and high-speed communication.
As a production method for the graded-index POF, there is a method in which a preform of the POF is formed and then heat-drawn to be the POF having a desired diameter. To apply the desired refractive index distribution to the preform, there is a method in which refractive index controlling agent is added in main content to be a light transmission section of the POF. By controlling the amount of the adding refractive index controlling agent according to the position in the radial direction, the refractive index can vary in the radial direction of the preform. For example, Japanese Patent Laid-Open Publication No. 2001-215345 discloses a method in which a monomer material is poured into a rotating tube and polymerized to be an outer shell, then the monomer material including the refractive index controlling agent is continuously poured into the tube while gradually increasing the adding amount of the refractive index controlling agent so as to form the light transmission section by polymerization.
However, it is difficult to make desired refractive index distribution with high accuracy by adding the refractive index controlling agent. In considering this problem, there is a method in which the light transmission section including a plurality of layers are formed by pouring polymerizable composition to be a layer on another layer containing residual (unreacted) polymerizable composition (for example, see Japanese Patent Laid-Open Publication No. 10-096825).
In Japanese Patent Laid-Open Publication No. 10-096825, some combination of the residual polymerizable composition in the layer and the other polymerizable composition poured on the layer possibly generates polymer blend which causes loss of the transparency of the light transmission section. In addition, when the difference between refractive indices of the adjacent two layers is not appropriately adjusted, the light entering into the light transmission section possibly reflected at an interface between the layers therein. Therefore, there is a need to find a production method for the POF having superior transparency and transmission band available to resent high-speed communication network, without above-described problems.
An object of the present invention is to provide an optical medium, which can be used for lenses or optical fibers including graded-index POF, and a production method of this optical medium.